This 200-201 practice question tests your understanding of network intrusion analysis. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
During incident response, a security analyst reviews a PCAP file and sees TCP packets with only the SYN flag set, followed by RST packets upon receiving a SYN-ACK. No connection is established. Which scanning technique is being used?
Half-open scan (SYN scan)
Half-open scan sends SYN, receives SYN-ACK, then RST to avoid detection.
FIN scan
Why wrong: FIN scan uses FIN packets, not SYN.
Christmas tree scan
Why wrong: Xmas scan sets FIN, PSH, and URG flags, not just SYN.
Full connect scan
Why wrong: Full connect scan completes the TCP handshake, but the observed pattern has no ACK.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Half-open scan (SYN scan)
The described behavior—sending a SYN packet, receiving a SYN-ACK, and immediately replying with an RST—is the hallmark of a half-open (SYN) scan. This technique never completes the three-way handshake, so the target does not log an established connection, making it stealthier than a full connect scan. The RST sent after the SYN-ACK terminates the handshake before it can be fully established, confirming the port is open without creating a full session.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Half-open scan (SYN scan)
Why this is correct
Half-open scan sends SYN, receives SYN-ACK, then RST to avoid detection.
Related concept
Read the scenario before looking for a memorised answer.
FIN scan
Why it's wrong here
FIN scan uses FIN packets, not SYN.
Christmas tree scan
Why it's wrong here
Xmas scan sets FIN, PSH, and URG flags, not just SYN.
Full connect scan
Why it's wrong here
Full connect scan completes the TCP handshake, but the observed pattern has no ACK.
Common exam traps
Cisco often tests the distinction between a half-open scan and a full connect scan by focusing on whether the three-way handshake is completed; the trap here is that candidates may confuse the RST sent after SYN-ACK as part of a normal connection teardown, rather than recognizing it as the defining characteristic of a SYN scan that never completes the handshake.
Detailed technical explanation
In a half-open scan, the scanner sends a SYN to the target port; if the port is open, the target responds with SYN-ACK, and the scanner immediately sends an RST to tear down the connection, never sending the final ACK. This technique exploits the fact that the target's TCP stack will allocate resources for the pending connection until the RST is received, but the connection is never fully established, so many applications and logging mechanisms do not record it. Tools like Nmap use this scan by default (with root privileges) because it is faster and less likely to be logged than a full connect scan, though modern intrusion detection systems (IDS) can detect the pattern of SYN followed by RST.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Network Intrusion Analysis — This question tests Network Intrusion Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: Half-open scan (SYN scan) — The described behavior—sending a SYN packet, receiving a SYN-ACK, and immediately replying with an RST—is the hallmark of a half-open (SYN) scan. This technique never completes the three-way handshake, so the target does not log an established connection, making it stealthier than a full connect scan. The RST sent after the SYN-ACK terminates the handshake before it can be fully established, confirming the port is open without creating a full session.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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